Existing telecommunications systems typically transmit optical signals over optical fibres with a pulse rate of 10 Gb/s and, for example, use wavelength division multiplexing to transmit eight channels simultaneously, thereby achieving a data rate of 80 Gb/s per fibre. Transmission of optical signals is possible over about 400 km, using appropriate optical amplifiers, before conversion to the electrical domain is required to effect further regeneration.
It is estimated that future requirements of long distance traffic will require a transmission capacity greater than 1 Terabit per second per fibre. This increase in performance cannot simply be accommodated in such systems by increasing the bit rate per channel because of the onset of non-linear effects such as self-phase modulation and because the associated electronic processing at increased serial data rates becomes prohibitively expensive. Similarly, increasing the number of channels per pulse presents difficulties. Currently each channel is provided by a separate laser source whose output is filtered to a respective channel wavelength, the wavelengths being separated by guard bands to provide suitable tolerance to drifting due to environmental effects or ageing and, coupled with the limited bandwidth of laser amplifiers used for optical repeating systems, a limitation therefore exists on the number of channels which can be added to expand the data handling capacity of the system.
It is known from De Souza et al, Optics Letters vol. 20, no. 10, p.1166-8, to provide wavelength division multiplexing using a single broadband femtosecond source by slicing the spectral bandwidth into 16 channels which are modulated individually. De Souza proposes using a diffraction grating and collimating lens to disperse light from the source onto a modulator array chip and to recombine the component wavelengths into an output beam for transmission to a receiver. A disadvantage of this technique is the size of the diffraction grating and associated optics.
A similar arrangement is proposed by Knox et al in U.S. Pat. No. 5,526,155 with the additional proposal that the diffraction grating could be replaced by a suitable wavelength splitter such as a Dragone wavelength router as described in C. Dragone, "An N.times.N Optical Multiplexer Using a Planar Arrangement of Two Star Couplers", IEEE Photonics Technology Letters, vol. 3, no. 9, pp812-815, September 1991. The Knox reference proposes that each channel is formed by many different longitudinal modes of the optical source, the output of a pulsed laser typically having a spectrum in which a series of closely spaced peaks, sometimes referred to as modes of the laser. It is proposed by Knox that 250 or more different channel signals may be generated in this manner to provide a transmission capacity of 25 Gbits/sec.
It is known from Shao et al, "WDM Coding for High-Capacity Lightwave Systems", Journal of Lightwave Technology, vol. 12, no. 1, January 1994, to provide error detection and correction coding in a wavelength division multiplexed optical system where n channels are provided by separate sources of respective wavelengths and parallel word transmission occurs such that n=k+r where k equals the number of data bits per word and r equals the number of parity bits per word. A Hamming coding scheme is utilised to define the calculation of parity bits and the data recovery process at the receiver.